The present invention relates to satellite communication systems. In particular, the present invention relates to bandwidth allocation in a downlink of a satellite communication system.
Today, relatively simple communications satellites provide information distribution on a global scale. While presently available communications satellites are effective, they generally have relatively little data processing capability or intelligence. However, technology and cost considerations have brought the communications industry to the point where future communication satellites will intelligently switch, process, and retransmit individual data cells. In such a satellites, a cell switch is the component of primary importance.
Unlike a cell switch for a terrestrial network (which simply routes cells between ports to users who are all considered to have equal ability to decode or read the cells), a space based cell switch has several unique requirements. An output port for a space based cell switch generally corresponds to a specific downlink beam supporting multiple terminals. Different terminals may experience different signal strengths due to weather, position in the beam, signal interference, or other factors. Thus, the signals transmitted to some terminals may require compensation such as additional error coding to ensure that the terminal receives the signal reliably. Over-coding where it is not necessary, however, wastes bandwidth and decreases the information throughput of the satellite (and therefore decreases its revenue potential).
Furthermore, terminal density is generally not uniform within the downlink beam. Thus, it may be more efficient to hop the downlink beam to different geographic locations to provide communications bandwidth for a large geographic area. In such a case, the cell switch would need to intelligently route cells to the downlink beam when the downlink beam was pointing at the geographic area for which the cell is destined. In addition, a mechanism for allocating bandwidth among the many terminals receiving data in the downlink beam is, of course, generally desirable.
As noted above, however, past terrestrial cell switches treated all receivers alike. In other words, the technology present in past terrestrial cell switches was inappropriate for a sophisticated cell switching satellite, in part due to the unique considerations of the space based environment explained above.
There is a need for an effective and flexible mechanism for cell switching and downlink bandwidth allocation in a satellite communication system.
It is an object of the present invention to provide a mechanism for downlink bandwidth allocation in a cell switching satellite.
Another object of the present invention is to allocate bandwidth in a downlink using a table based technique.
Yet another object of the present invention is to provide a mechanism for scheduling downlink transmission of data cells according to predetermined cell subclasses.
One or more of the foregoing objects is met in whole or in part by a method for allocating bandwidth in a satellite downlink. The method allocates subclass entries in an ordered list (which may be organized as a table, for example). Each entry generally corresponds to one of several predetermined cell subclasses, and the allocation reserves a total number of each subclass entry in the ordered list according to a predetermined amount of bandwidth desired for each cell subclass.
Thus, for example, all, some, or none of the bandwidth available in the downlink may be reserved for a first subclass of cells, while the remaining bandwidth may then be used for a second subclass of cells. The subclasses may be associated with factors such as coding rate, downlink beam area, virtual paths or virtual circuits, quality of service parameters, and the like. The method then examines each subclass entry in the ordered list and selects a cell from a cell memory (which may be a queue) that matches the subclass entry. The matched cell is then transmitted.
The method may also allocate sets of subclass entries in groups corresponding to a frame size. For example, downlink bandwidth for a downlink comprised of 16 slot (or any other size) frames may be provided by selecting sets of 16 subclass entries mutually compatible for transmission in a single frame. As one example, each of the 16 entries may share a common coding rate and quality of service.
The present invention also provides a downlink bandwidth controller for allocating bandwidth in a downlink. The downlink bandwidth controller includes a list memory storing an ordered list of subclass entries. Each subclass entry generally corresponds to one of several predetermined cell subclasses. In addition, the ordered list includes a sum of subclass entries which indicates the predetermined amount of bandwidth desired for each cell subclass. Also included is cell selection circuitry coupled to the list memory for extracting a cell from a cell memory that matches a subclass entry under examination in the ordered list. A transmitter is provided to transmit each extracted cell.
In the downlink bandwidth controller, the list memory may be stored as a table and the cell memory may be stored as a queue, for example. In one embodiment, the cell memory and the list memory are common portions of a main memory, although the cell and list memories may be implemented separately. As noted above, the subclass entries may represent coding rates, downlink beam areas, quality of service parameters, and the like.